The objective of the present study was to examine cases of acute aortic dissection in order to analyze the clinical and diagnostic findings, and to summarize their treatment modalities, as well as their hospital outcomes. Between July 1998 and June 1999, we prospectively studied patients who were newly diagnosed as having acute aortic dissection at 25 hospitals in Mie prefecture. These cases were examined for their demographics, the characteristics of the clinical findings, diagnostic methods, treatment modalities according to the type of aortic dissection, and the early morbidity and mortality of the hospital outcomes. Of 66 newly diagnosed aortic dissections (43 males), 30 were type A and 36 were type B. Seventy-six percent of the cases arrived at a medical facility within 6 hours from the onset of symptoms. Frequent initial symptoms and clinical findings were pain in 95.5%, cardiac arrest and/or hypotension in 21%, pericardial effusion in 29%, pleural effusion in 25%, and neurological signs in 30%. Twenty-one patients underwent surgical repair, 36 were treated medically, and 5 underwent endovascular stenting. Overall early mortality was 12.1%, which included 2 DOA. Fifty percent of these deaths occurred within 48 hours, and 63% by 72 hours of the initial event. In spite of the relatively rare incidence of acute aortic dissection in our study, the calculated incidence was 4.0/100,000/year. The overall mortality rate was relatively low compared to the figures reported in the literature, suggesting the earliest possible diagnosis and timely intervention are critically important to attain successful outcomes.